#include <stdio.h>

/*int main(int argc,char *argv[])
{
	int x;
	scanf("%d",&x);
	
	int sun = 0;//总和
	int i = 1;//起始值
	
	while(i <= x)      
	{
		sun += i;
		i++;
	}
	
	printf("sun = %d\n",sun);
	
	return 0;
}
*/
int main(int argc,char *argv[])
{
	int a,b = 0,c = 1;//a是输入值，b总和，c累加值
	scanf("%d",&a);
	while(c <= a)
	{
		b += c++;//第一次b=1，c=2 //第二次b=3，c=3//第三次b=6，c=4//第四次b=10，c5
	}
	printf("sum = %d\n",b);
	
	
	return 0;
}